A single phase series motor is also known as a universal motor as it can operate on either AC or DC power. It is a single phase motor with a commutator whose rotor windings are connected in series with stator windings via brushes. Due to its characteristics of high rotating speed, small size, high starting torque, convenient speed regulation and series excitation, the universal motor is commonly used for driving low power appliances that need a high rotating speed, small size and low weight, such as medical devices, power tools, domestic electric appliances and so on.
A typical known universal motor comprises a stator core which comprises a yoke with a rectangular configuration and two teeth (stator poles) which extend inwardly from two opposing sides of the yoke. Each tooth has a winding wound thereon. When the windings are electrified, only two magnetic poles with opposite polarity are formed on the stator, thereby the range of speed regulation of the motor is small.